1. Field of the Invention
The present invention relates to the field of power plant technology. It pertains to a method for operating a (stationary) gas turbine as well as to a gas turbine for implementing the method.
2. Brief Description of the Related Art
A gas turbine with reheating (reheat gas turbine) is known (see, for example, the U.S. Pat. No. 5,577,378 or “State-of-the-art gas turbines—a brief update,” ABB Review February 1997, FIG. 15, turbine type GT26), which combines flexible operation with very low flue gas emission values.
The machinery architecture of the gas turbine of Type GT26 is unique and is exceptionally well-suited to realizing a concept that is the subject matter of the present invention, because:                even in the case of the compressor, there is a significant diversion of compressor air at intermediate compressor pressures,        the concept of sequential combustion renders an increased stability of combustion possible in conjunction with reduced levels of excess oxygen, and        a secondary air system is present, which renders it possible to divert air from the compressor, to cool it down, and to use the cooled air for cooling the combustor and the turbine.        
The principle of the known gas turbine with reheating is shown in FIG. 1. The gas turbine 11, which is a portion of a combined cycle power plant 10, includes two connected compressors, arranged behind one another on a commonly shared shaft 15, namely a low pressure compressor 13 and a high pressure compressor 14, as well as two combustors, namely a high pressure combustor 18 and a reheat combustor 19, and the pertinent turbines, namely a high pressure turbine 16 and a low pressure turbine 17. The shaft 15 drives a generator 12.
The manner in which the unit works is as follows: air is drawn in from the low pressure compressor 13 via an air inlet 20, and is compressed initially to a level of intermediate pressure (ca. 20 bar). The high pressure compressor 14 then further compresses the air to a level of high pressure (ca.32 bar). Cooling air is diverted at both the level of intermediate pressure and at the level of high pressure and cooled down in pertinent OTC coolers (OTC=Once Through Cooler) 23 and 24 and conducted further to the combusters 18 and 19 and turbines 16, 17 for cooling purposes. The remaining air from the high pressure compressor 14 is led to the high pressure combustor 18, and heated there by the combustion of a fuel introduced by fuel feedline 21. The resultant flue gas is then expanded in the downstream high pressure turbine 16 to an intermediate level of pressure as it performs work. After expansion, the flue gas is heated again in the reheat combustor 19 by the combustion of a fuel introduced by fuel feedline 22 before it is expanded in the downstream low pressure turbine 17, performing additional work in the process.
The cooling air, which flows through the cooling lines 25, 26, is sprayed in at suitable points of the combustors 18, 19 and turbines 16, 17 to limit the material temperatures to a reasonable degree. The flue gas coming out of the low pressure turbine 17 is sent through a heat recovery steam generator 27 (HRSG) to generate steam, which flows through a steam turbine 29 within a water-steam circuit, and performs additional work there. After flowing through the heat recovery steam generator 27, the flue gas is finally given off to the outside through a flue gas line 28. The OTC coolers 23, 24 are part of the water-steam circuit; superheated steam is generated at their outlets.
Due to the two combustions in the combustors 18 and 19, which are independent of each other and follow each other in sequence, great operational flexibility is achieved; the temperatures in the combustors can be adjusted in such a way that the maximum degree of efficiency is achieved within the existing limits. The low flue gas levels of the sequential combustion system are the result of the inherently low emission levels that can be achieved in the case of reheating (under certain conditions, the second combustion even leads to a consumption of NOx).
On the other hand, combined cycle power plants with single stage combustion in the gas turbines are known (see, for example, U.S. Pat. No. 4,785,622 or U.S. Pat. No. 6,513,317 B2), in which a coal gasification unit is integrated in order to provide the requisite fuel for the gas turbine in the form of syngas, which is recovered from coal. Such combined cycle power plants are designated IGCC (Integrated Gasification Combined Cycle) plants.
The present invention now proceeds from the recognition that due to the use of gas turbines with reheating in an IGCC plant, the advantages of this type of gas turbine can be made usable for the plant in a particular manner.